I'd like an incoming caller to hear constant (low volume) background music while speaking with the called party. The called party (leg B) should not hear the music.

The music will come from another extension, for example, the tetris tone_stream extension in the vanilla configuration (link below), but will most likely use `endless_playback` instead in practice.

I suspect I could achieve this with `eavesdrop` and `eavesdrop_whisper_aleg=true` but I can't get my head around how to express this in the dialplan XML.

For the purpose of this question, the delay_echo extension in the vanilla conf could be an example of an extension that a caller would dial. A successful end result here would be the caller would hear the tetris tones and an delayed echo of anything the caller said, but the music would not be echoed.

For clarity, I'd to find a way for this to work for human-to-human calls also.